1. Field of the Invention
The invention relates generally to high intensity metal halide lamps and, particularly, to such lamps with improved structural and mechanical integrity.
2. Description of the Prior Art
High pressure mercury vapor discharge lamps with and without additional chemical additives are well-known in the prior art (for example, see U.S. Pat. No. 3,654,506 issued to Kuehl, et al.). Such lamps are commonly constructed from vitreous quartz tubing which contain electrode subassemblies. In the typical high intensity metal halide lamp, a vitreous quartz lamp envelope is filled with argon gas, mercury, plus other metal salts. Protruding into the lamp envelope are two tungsten electrodes, each electrode being connected to molybdenum foil which is in turn connected to a follower rod, which serves as an electrical termination. The tungsten electrode assemblies including the molybdenum foil are generally, but not always, concentric with the axis of the lamp envelope and located at opposite extremes of the lamp envelope. Each tungsten electrode assembly, including the molybdenum foil is encased in a vitreous quartz tube. While the discharge is actually struck between the tungsten electrodes within the sealed lamp envelope, high electrical currents are conducted to the discharge envelope through the follower rod and molybdenum foil conductors. In addition to conducting electrical excitation to the lamp envelope, the quartz arms containing the molybdenum foil attached to the tungsten electrodes also provide the mechanical support for the lamp envelope.
It has been found that lamps of this high intensity type often fail if the seal between the vitreous quartz and the molybdenum foil is not absolutely perfect. That is, in manufacture of such lamps, intimate contact between the molybdenum foil and the vitreous quartz tubular arm must be assured. If a gap or space exists between the molybdenum foil and the vitreous quartz, a high probability exists that a crack will be formed within the vitreous quartz due to the high currents flowing within the molybdenum foil causing incipient failure of the lamp.
Manufacturing processes and structures have been suggested in the prior art to achieve a molybdenum ribbon/vitreous quartz high integrity seal. One commonly used sealing method is the pinch or press seal. In the pinch or press operation the electrode ribbon assembly is supported within a thin wall quartz tube. A two-part ring burner heats the quartz tubing to soften it, and a pair of opposing pinch jaws strike the soft quartz tubing and seal it firmly to the ribbon. Because a pinch seal is made in a single operation, its maximum length is limited to an inch or less. Also, a pinch seal has little mechanical strength and cannot be used to support the relatively heavy discharge envelope.
To make a molybdenum ribbon seal air compatible it is necessary to make it long enough to insure that the end exposed to air is operating at a temperature low enough to preclude oxidation of the molybdenum quartz interface, or to provide a second seal which prevents the oxidizing atmosphere from reaching the seal.
These problems are successfully addressed in U.S. Pat. No. 3,205,395 issued to Buchwald. Buchwald teaches the construction of a high intensity lamp by using a quartz stem pressed tube incorporating a second seal to prevent oxidation of the molybdenum ribbon seal. Moreover, Buchwald teaches the insertion of the stem pressed tube into a vitreous quartz tube arm of slightly larger inside diameter than the outside dimension of the stem pressed tube. This built up assembly provides the mechanical structure which constitutes the arm attached to the lamp envelope. A gap thus results between the quartz of the stem pressed tube and the exterior quartz arm. This discontinuity in quartz is not beneficial to the performance of the lamp reducing the structural integrity of the lamp arm while not providing the heat dissipation needed for high current operation.
In an attempt to form an improved seal between the molybdenum foil and the vitreous quartz, vacuum shrinking of the quartz arm onto the molybdenum foil electrode assembly is known in the art. Typically, an electrode subassembly consisting of the tungsten electrode attached to the molybdenum foil in turn attached to the electrical terminator is inserted within a relatively thick walled vitreous quartz tube. Using techniques known in the art, the vitreous quartz tube is vacuum shrunk about the molybdenum foil assembly upon application of suitable heat. This is clearly an improvement over the stem pressed structure as taught by Buchwald since this structure can be made long enough to cause the exposed end to operate below the oxidation temperature of the molybdenum ribbon, thereby eliminating the necessity of a second seal. However, it has several practical drawbacks. To achieve the strength necessary to support the discharge envelope, the thickness of the vitreous quartz which must be shrunk onto the molybdenum foil is on the order of three millimeters and it is extremely difficult to heat such a thick-walled cylinder uniformly due to slight variations in the wall of the quartz tube. Because of the poor heat conducting properties of quartz a large temperature gradient appears between the outside surface of the tube where heat is applied and the inside surface where the seal is to be made. A temperature well in excess of the softening point is needed at the inside wall to insure that the quartz flows well onto the ribbon and into all voids. To achieve this flow on the inside surface requires that the outside surface be molten and extremely free flowing. The heating and vacuum shrinking of such thick-walled vitreous quartz about the molybdenum foil is extremely delicate, and it has been performed in practice only by skilled artisans through manual operation. Due to the delicacy of the operation, is has not been possible to automate same. A further problem which plagues seals incorporating heavy side arms is residual strain left in the quartz subsequent to the sealing. Although annealing removes a good portion of the strain, subsequent thermal cycling caused by operation of the lamp can introduce new strains in the side arms. These strains can build to the point of causing a crank in the side arm which invariably destroys the integrity of the seal and causes the lamp to fail. Even utilizing handmade techniques which are extremely expensive, high intensity lamps of the type manufactured by heat and vacuum shrinking thick-walled quartz about the molybdenum foil often fail due to molybdenum foil/vitreous quartz seal defects.